The invention relates to a system for mooring a tanker near an offshore production unit which is anchored by means of a number of anchor lines and associated anchors, and from which a tanker in use of the system is supplied with oil or gas via a transfer means, the tanker normally being moored behind the production unit, as seen in the prevailing weather direction.
In connection with offshore production of hydrocarbon products (oil and gas) in regions with calm weather conditions (for example the coast of Western Africa and Brazil), it is common to use floating production vessels with conventional spread mooring. Such an arrangement is shown in FIG. 1 of the drawings, wherein a production vessel 1 is anchored by means of a spread mooring comprising a plurality of anchor lines 2 extending between the production vessel 1 and respective non-illustrated anchors.
In the following description there is referred to FPSOs (Floating Production Storage Offloading) which is the branch designation of production vessels of the relevant type. If the unit is also equipped for drilling, the branch designation is FDPSO (Floating Drilling Production Storage Offloading).
In connection with the topical loading operations, the production vessel (FPSO) 1 will be oriented with the bow against the prevailing weather/current direction which is represented by the arrow W in FIG. 1.
In the known system according to FIG. 1 the transport of the crude oil produced on board the FPSO 1 takes place by means of tankers 3 which typically moor at the stern of the FPSO (tandem mooring) and receive the load via a floating hose 4 which is connected to the midship manifold 5 of the tanker.
One or more smaller tender vessels 6 normally will assist during mooring and disconnection of the tanker 3. During the loading operation proper one or more tender vessels 6 possibly will also be able to be used to keep the tanker 3 at a desired position in relation to the FPSO 1.
Experience has shown that the tanker 3, which during a tandem loading operation typically is situated only about 50-100 m from the stern of the FPSO 1, may have the ill-luck to touch/collide with this vessel. Among other things, this may happen when the current direction changes 180xc2x0 in the course of a short time. The consequences of such incidences may be very serious.
The oil industry therefore has also chosen other mooring solutions for the tanker 3, where the solutions afford a greater security, but simultaneously are more expensive.
A typical such solution is shown in FIG. 2 which shows a xe2x80x9cspread mooringxe2x80x9d anchored FPSO having a so-called Calm buoy 7 for loading/export to a tanker 3. The solution implies that the tanker 3 moors to the Calm buoy 7 which is located at a xe2x80x9csafexe2x80x9d distance L1 (typically 2-4 km) from the FPSO 1. The tanker 3, which then normally only uses a mooring hawser 8 at the bow, will be able to turn around the Calm buoy 7 under the influence of varying environmental forces, the buoy being provided with a swivel 9 allowing said turning.
However, such a system implies large extra costs which, among other things, are due to the fact that risers and a pipe line 10 must be installed between the FPSO vessel 1 and the Calm buoy 7. Particularly the costs will be high if the water depth is great.
The object of the invention is to provide a mooring system for the stated purpose affording a safe mooring of the tanker near an offshore production unit, and where the introductorily mentioned problems are taken care of in a simple and cost-efficient manner.
For achieving the above-mentioned object there is provided a system of the introductorily stated type which is characterised in that it comprises a first and a second pair of bottom-anchored buoys located before or behind the production unit, as seen in the prevailing weather direction W, and at a chosen distance from the production unit, the buoys of the first pair being located closer to the production unit than the buoys of the second pair, the distance between the two buoy pairs being greater than the length of the tankers to be moored, and the buoys of each pair having a chosen mutual distance transversely to said weather direction, the buoys being arranged for connection of respective mooring lines for mooring of a tanker in the region between the buoys with the fore-and-aft direction essentially coinciding with said weather direction, and that it further comprises a collision barrier extending between the buoys of the first buoy pair and being connected thereto, to prevent a collision between the tanker and the production unit.
The above stated system can be used both in connection with a xe2x80x9cspreadly mooredxe2x80x9d FPSO as described above, and other fixedly anchored platfornms constituting offshore production units. The system makes use of only known technology and maritime operational procedures.
It is also an object of the invention to provide a mooring system affording a safe mooring of a tanker near a floating unit, such as a xe2x80x9csparxe2x80x9d platform, where the introductorily mentioned problems are taken care of in a simple and cost-efficient manner.
For achieving the above-mentioned object there is provided a system for mooring a tanker near a floating unit which is anchored by means of a number of anchor lines and associated anchors, and from which a tanker in use of the system is supplied with oil via a transfer means, the tanker being moored behind the floating unit as seen in the prevailing weather direction, which system is characterized in that it comprises a first pair of bottom-anchored buoys located behind the floating unit, as seen in the prevailing weather direction, and at a chosen distance from the floating unit, the buoys having a chosen mutual distance transversely to the weather direction and being arranged for connection of a respective end of a pair of mooring lines for mooring of a tanker by connecting the other ends of the mooring lines to the bow of the tanker, and that it further comprises a collision barrier extending between the buoys and being connected thereto, to prevent a collision between the tanker and the floating unit.